Target compounds

The second stage of oxidation following the formation of hydroxyl radicals involved in AOPs is the reaction of these formed species with the target compounds in water. The choice of the target compounds was motivated by the efficient capability of this technology to degrade refractory compounds that are ubiquitous in the environment and may pose danger to aquatic life.

2.4.1 Phenol

Phenol is considered to be a reference compound for evaluating the effectiveness of an AOP as it is among the most widespread toxic water-soluble persistent chemicals that has acute environmental impact. Effluents from many industries such as petroleum refineries, coke plants, chemical plants, explosive producers and phenolic resin manufacturers are generally known to contain high levels of phenolic compounds. These substances are prevalent, and the fact that they are known as hazardous and toxic to aquatic life means that they would have to be removed from concentrated wastewaters prior to release in the aquatic environment. Thus this chemical has been intensely studied for decades, accumulating a substantial amount of publicly available data for reference. Phenol is also in general a rapidly oxidizable substance allowing studies of fast reaction under various operating conditions of treatment, for example, pulse repetition frequency.

2.4.2 Pharmaceuticals

The increasing number of analytical techniques developed for detecting minute levels of compounds in water samples have led to a whole new perspective of how we deal with the disposal and use of medical drugs. The presence of prescription and non-prescription drugs and their metabolites in surface water streams studied in North America and Europe are drawing attention not only for the unknown effects of these compounds to human and aquatic life when ingested, but also for the apparent inefficiency of the wastewater

treatment methods being used in the sewage treatment plants (STPs). Some of the pharmaceuticals in this study are among the highly consumed drugs in the households today. Genotoxic as well as immunotoxic effects to fishes have been reported after exposure to wastewater containing estrogenic and alkylphenolic chemicals (Liney, et al., 2006).

-Estradiol

An estrogen or a female hormone, estradiol is used for hormone replacement therapies to treat postmenopausal symptoms. Another derivative of estradiol, ethinyestradiol, is one of the main ingredients of hormonal contraceptives. The occurence of these compounds (Huang & Sedlak, 2001) in sewage treatment plant effluents are at levels that could cause feminization of wild fish such as perch or trout (Kavanagh, et al., 2004), (Bjerregaard, et al., 2006). Constant exposure to these hormonally active chemicals would affect the sexual behaviour and immune function of fish. The detectable presence of estradiol in receiving bodies of water near STPs is a concern that needs to be addressed by the applied wastewater treatment technique. The presence of highly potent hormones in surface waters could be one of the inevitably growing challenges in environmental science and technology.

Paracetamol

A widely used over-the-counter pain killer and fever reducer, paracetamol (acetaminophen in North America) is also one of the main active pharmaceutical ingredients in flu medications, making it a very common household drug. Consumed in high amounts, it is not surprising that the presence of this compound in surface waters is of highest concentration relative to other pharmaceutical compounds (Vulliet, et al., 2011).

Indometacin

Indometacin is another non-steroidal anti-inflammatory medication prescribed for fever, swelling and pain, although considered to be more potent than paracetamol. Another clinical use of indomethacin is to delay premature labour by reducing uterine contractions.

Ibuprofen

Another popular over-the-counter pain reliever, ibuprofen is among the most popular drugs in the world, and the second most consumed drug for the musco-skeletal subgroup in Finland as of 2012 (FIMEA, 2012) despite being intended for short-term use and temporary treatment. The presence of ibuprofen in rivers and surface waters has been detected (Ternes, 1998) along with other pharmaceuticals, suggesting its high mobility in aquatic environment (Buser, et al., 1999).

Salicylic acid

Salicylic acid is another anti-inflammatory drug known for relieving fever and easing aches. It is also typically a constituent of topical liniments for soothing muscle pains, and can be found in many skin-care products for treating acne or dermatitis, and in shampoos for

minimizing dandruff. Due to wide usage of products containing salicylic acid, it is most likely that this compound is highly present in the wastewater streams.

2.4.3 Lignin

Lignin is an organic compound mostly derived from wood, and most plants such as straw. It is commonly found in pulp and paper manufacturing wastewater as a dissolved compound when it is separated during the pulping process. Lignin contains various phenolic and non-phenolic aromatic structural elements formed during its biosynthesis by dehydrogenative polymerization of coniferyl and synapyl alcohols. It comprises about 10-15% by weight of black liquor which is the major pulp and paper side stream. In waste water streams, its presence causes high concentration of dissolved organic matter, dark brown colour, and odour. Conventional treatment processes are not effective in destroying lignin: coagulation can only remove large molecules of lignosulfonates (Dilek & Gokçay, 1994); increased chlorination for the treatment of potable water containing lignin can lead to formation of hazardous chlorinated substances (Chang, et al., 2004). With increasingly stringent requirements for discharges, it is important to have cost effective methods capable of degrading lignin.

Lignin also presents an interest as a bulk source of valuable organic raw material that is able to substitute fossil based raw materials for plastics, carbon fibres or for certain chemical products. For instance, a study by Gooselink, et al. (2011) on lignin valorisation shows a potential of lignin oil as a replacement for phenol in a PF-wood resins.

2.4.4 Humic Substances

Humic substances (HS) comprise a major portion of dissolved natural organic matter such as microbially degraded plant tissues including lignin. They are evident in the brown coloration of surface waters and some water supply systems as they are inherently difficult for microbes to mineralize. Their inherent recalcitrance makes biodegradation under natural conditions insufficient for mineralization, and as such, HS remain in the surface waters or in water supply systems if not removed by physical methods (filtration, coagulation, adsorption). During treatment, disinfection by chlorination of water containing these substances can lead to the formation of trihalomethanes (THMs) which is a hazardous by-product that increases adverse birth risks (Gallagher, et al., 1998).

A summary of the target compounds used in this study and the analytical techniques used to measure the parent compound is demonstrated in table 2 below. The analyses of

intermediate products are described in more detail in Publications I-IV.

Table 2. Analytical methods for measurement of parent compound.